1. Field of the Invention
The present invention relates to a remote return loop control system in a data transmission system having data transmission units such as data modulators/demodulators (hereinafter referred to as modems) connected at opposite ends of a transmission line, in which a return loop is formed at the remote data transmission unit by a command from the other data transmission unit when the transmission line, modulator, demodulator, etc. are to be tested.
2. Description of the Prior Art
A prior art remote return loop control system of this type usually uses a signal in a frequency range beyond the frequency band used for the modem or a predetermined combination of code sequences as a loop command signal. The former method, however, needs accurate oscillator and band-pass filters and hence has an economic problem. In addition, when the transmission rate is low, the frequency band of the modem is wide and hence it is difficult to transmit the loop command signal. In the latter method, if code sequences which are identical to those of the loop command signal occur during normal data transmission, they may be detected as the loop command signal and the return loop may be formed. Therefore, restriction is imposed on the degree of freedom in the code of data the transmission of which is desired.